I can't remember changing anything significant (only Thunderbolt Firmware-Update and a few minor App-updates), but fortunately the problem disappeared. I'm sorry for you guys, but I have no explanation and therefore no suggestion how to solve the problem for you.

I have also been having this problem. I managed to solve it by disconnecting the ethernet cable (I have a wired connection), then reconnecting. I suspect turning off wireless in the case of an over the air connection, then re-enabling it may do the same.

Changed my DNS to 8.8.8.8 (suggested somewhere on this site) for some other reason (my router's and ISP's DNS services not being very good I think) and hey presto fixed this at the same time.

I had resisted doing this but it's not too hard if you are prepared to dive into Settings --> Network --> Wi-Fi (or maybe Ethernet if you have hard-wired connection) --> Advanced... button bottom right --> DNS --> Press the + underneath the "DNS servers" pane, bottom left, and type in 8.8.8.8 --> OK --> Apply

Mail (and internet browsing) now working much better for a couple of weeks now

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